* This commit only covers the migration of the icons in the menu and draftail editor. All
other admin menu icons remain implemented as before.
* Existing font based icon support still exists.
* Convert menu icons to SVG
* This could be considered a breaking change for any Wagtail sites or
packages that are leveraging Wagtail's Button and Icon React components.
* Remove references to unused `submenu-trigger` class
* Prefix icon `id`s with `icon-` to prevent `id` collisions
* Add hooks support (not yet documented) for adding custom icons
